1. Field of the Invention
The present invention relates to micro contact printing on substrates. More specifically, the invention relates to hydrophilic silicone elastomer stamps and to the manufacture of such stamps useful in micro contact printing.
2. Background of the Invention
It is often a goal in areas such as electronics, organic and inorganic chemical catalysts, and electrochemistry to rapidly and economically apply to surfaces chemical activating agents. Particularly in the fabrication of microelectronic devices, optics, integrated circuits and the like, it is desirable to deposit patterned metal pathways on a surface in an economically and environmentally sound manner.
Micro contact printing (hereinafter xc3x81xcexcCP) is a technique for forming patterns of organic monolayers with micrometer and submicron lateral dimensions. It offers experimental simplicity and flexibility in forming certain types of patterns. Conventionally, such techniques rely on the remarkable ability of self-assembled monolayers of long chain alkanethiolates to form on, e.g., gold or other metals. These patterns can act as nanometer resists by protecting the supporting metal from corrosion by appropriately formulated etchants, or can allow for the selective placement of fluids on hydrophilic regions of the pattern. Patterns of self-assembled monolayers having dimensions that can be less than 1 micrometer are formed by using the alkanethiol as an xe2x80x98inkxe2x80x99, and by printing them on the metal support using an elastomeric xe2x80x98stampxe2x80x99. The stamp is fabricated by molding a silicone elastomer using a master prepared by optical X-ray microlithography or by other techniques. Patterning of the surface of such a stamp is, for example, disclosed by Biebuyck et al. in U.S. Pat. No. 5,925,259.
All demonstrations and applications of xc3x81xcexcCP so far use patterned stamps which are formed by polymerization of a polydimethylsiloxane (PDMS) elastomer in a mold. After curing, the PDMS stamp retains the negative of the pattern of its mold and can be used for series of inking and printing. Stamps represent a critical object for xc3x81xcexcCP comparable to optical Cr/quartz masks for photolithography. For this reason, stamps should carry an accurate pattern, be stable during printing, inking, and peeling off from surfaces (e.g. the mold) but they should be soft enough to intimately contact the substrate onto which they are applied. Stamps should additionally be easy to fabricate and manipulate. Silicone elastomers as a stamp material fulfill the above mentioned requirements almost perfectly.
However, silicone elastomer stamps useful in xc3x81xcexcCP suffer from an essential drawback. Their surface, as their bulk, is hydrophobic and can only uptake non polar chemical species present in the ink. Consequently, this restricts the application of xc3x81xcexcCP essentially to non polar inks such as alkanethiols and printing them on metallic substrates such as gold, silver or copper. The fabrication of stamps with hydrophilic surfaces would considerably expand the scope of applications of xc3x81xcexcCP to inking and printing many types of chemicals on a possibly large variety of substrates provided that these stamps would be capable of conformal contact with substrates and carry an accurate pattern. Inking and printing polar catalysts for electroless deposition of Cu, for example, is impossible with hydrophobic PDMS stamps. PDMS stamps have first to be hydrophilized with an O2xe2x80x94 based plasma before inking them with the solution with a solution containing the polar catalysts. This plasma treatment generates, however, a layer on the surface of the stamp which is unstable, thin with an uncontrolled thickness, hydrophilic without any control, and brittle. A second method to hydrophilize stamps applies strong oxidants like chromic acid (Cr2O32xe2x88x92) or permanganate (MnO4xe2x88x92) to a self-assembled monolayer on the surface of PDMS. Like for the first method, the oxidized layer is thin. Such strong oxidation treatments can lead to rough PDMS surfaces which compromise good contact between the stamp and the substrate during printing.
In summary, hydrophilization of stamps should be done in silicone elastomers because it is still the optimal material for fabricating stamps for micro contact printing. This hydrophilization should be as simple as possible, homogeneous and lead to high and controllable yields.
Preferably, the stamps should be hydrophilic on their surface with a good control of both the chemical composition and the thickness of the hydrophilic layer. If the hydrophilic layer is too thin, hydrophobic low-molecular-weight components from the PDMS bulk can migrate through the hydrophilic layer to the surface and cover it and/or the hydrophilic layer can xe2x80x98reconstructxe2x80x99 towards the bulk of the stamp to minimize its surface free energy. Additionally, too thin hydrophilic layers may not be able to incorporate enough polar ink for well printed patterns. Control over the chemical composition of the hydrophilic layer is equally important to maximize uptake of the relevant species from the ink and to prevent or minimize unwanted side reactions. The hydrophilic layer should also be grafted or deposited employing a chemistry that does not damage the stamp and preserves the accuracy of its dimensions and micropattern.
Alternatively, the entire stamp could incorporate hydrophilic constituents or it could be hydrophilic in bulk. In this case, stamps may prove susceptible to uncontrolled or to strong swelling by polar inks and thus to distortion of their micropattern.
Up to now, an oxygen plasma treatment was sufficient to render the PDMS surface hydrophilic just before use, and a prolonged stability of the hydrophilic layer was not required. Plasma treatment of PDMS surfaces is, e.g., disclosed in Gregory S. Ferguson et al., xe2x80x98Monolayers on Disordered Substrates: Self-Assembly of Alkyltrichlorosilanes on Surface-Modified Polyethylene and Poly(dimethylsiloxane)xe2x80x99, Macromolecules, Vol. 26, No. 22, 1993, pages 5870 ff. and Manoj K. Chaudhury et al., xe2x80x98Correlation Between Surface Free Energy and Surface Constitutionxe2x80x99, Science, Vol. 255, 6 Mar. 1992, pages 1230 ff.
It is therefore an object of the present invention to provide hydrophilic stamps for micro contact printing that remain stable over a long period of time.
It is another object of the present invention to provide such hydrophilic stamps where the hydrophilization does not distort the pattern on the stamp.
These and other objects and advantages are achieved by the manufacturing methods and PDMS stamp defined in the independent claims appended hereto.
Preferred embodiments of the invention are described in the dependent claims.